Magnetic wire-based sensors for the micro-rheology of complex fluids

We propose a simple micro-rheology technique to evaluate the viscoelastic properties of complex fluids. The method is based on the use of magnetic wires of a few microns in length submitted to a rotational magnetic field. In this work, the method is implemented on a surfactant wormlike micellar solution that behaves as an ideal Maxwell fluid. With increasing frequency, the wires undergo a transition between a steady and a hindered rotation regime. The study shows that the average rotational velocity and the amplitudes of the oscillations obey scaling laws with well-defined exponents. From a comparison between model predictions and experiments, the rheological parameters of the fluid are determined.Comment: 14 pages 7 figures, accepted in Physical Review

Intracellular micro-rheology probed by micron-sized wires

In the last decade, rapid advances have been made in the field of micro-rheology of cells and tissues. Given the complexity of living systems, there is a need for the development of new types of nano- and micron-sized probes, and in particular of probes with controlled interactions with the surrounding medium. In the present paper, we evaluate the use of micron-sized wires as potential probes of the mechanical properties of cells. The wire-based micro-rheology technique is applied to living cells such as murine fibroblasts and canine kidney epithelial cells. The mean-squared angular displacement (MSAD) of wires associated to their rotational dynamics is obtained as a function of the time using optical microscopy and image processing. It reveals a Brownian-like diffusive regime where the MSA scale linearly with time and as the inverse of the cube of the wire length. This scaling suggests that an effective viscosity of the intracellular medium can be determined, and that in the range 1 - 10 micrometers it does not depend on the length scale over which it is measured.Comment: 13 page, 5 figures, accepted at Biomaterial

3D rotational diffusion of micrometric wires using 2D video microscopy

We propose a simple way to measure the three-dimensional rotational diffusion of micrometric wires, using two-dimensional video microscopy. The out-of-plane Brownian motion of the wires in a viscous fluid is deduced from their projection on the focal plane of an optical microscope objective. An angular variable reflecting the out-of-plane motion, and satisfying a Langevin equation, is computed from the apparent wire length and its projected angular displacement. The rotational diffusion coefficient of wires between 1 and 100 μm is extracted, as well as the diameter distribution. Translational and rotational diffusion were found to be in good agreement. This is a promising way to characterize soft visco-elastic materials, and probe the dimension of anisotropic objects

Métallo-Carbènes : Apoptose et arrêt du Cycle Cellulaire

Communication oral

The alternatives to soybeans for animal feed in the tropics

International audienceThere are many alternatives to soya for animal feed in the tropics (e.g. meals coming from local protein-rich resources such as beans, peas, aquatic plants or leguminous foliage). The study and evaluation of such alternatives must be based on their amino acid availability and profile, using the approach of investigating non-ruminant species. For this criterion, there is often a gap between the amino acid profiles of plant resources and the profile of amino acids truly available for the animal. In the short and medium term, new studies have to be performed to take into account the large diversity of rich protein resources in the tropics. Overall, the alternatives are broader for herbivores than for other animal species, since the concentration of fibre and secondary compounds is a limiting factor that will discriminate their proposal of use among domestic animal species. Such evaluations must also take into account the farming system functions and productive purposes. The paradigm is changing and, compared to years ago, maximising animal performance is not the priority goal of the systemic approach. Currently, multiple animal responses to alternative diets should be taken into account for their optimisation. A criterion such as environmental impact is often decisive in the combination of global and local approaches

3D rotational diffusion of micrometric wires using 2D video microscopy

We propose a simple way to perform three-dimensional (3D) rotational microrheology using two-dimensional (2D) video microscopy. The 3D rotational brownian motion of micrometric wires in a viscous fluid is deduced from their projection on the focal plane of an optical microscope objective. The rotational diffusion coefficient of the wires of length between 1-100 \mu m is extracted, as well as their diameter distribution in good agreement with electron microscopy measurements. This is a promising way to characterize soft visco-elastic materials, and probe the dimensions of anisotropic objects.Comment: 10 pages, 7 Figures, accepted for publication in EP